Although an automatic transmission is less efficient in terms of fuel economy than a manual transmission, more and more cars are equipped with the automatic transmissions.
In general, an automatic transmission includes a hydraulic torque converter, a gear train having planetary gear sets, friction elements for engaging/disengaging various members of the gear train, a hydrostatic oil pump supplying a hydraulic pressure to the torque converter and the friction elements, and hydraulic/electronic control devices.
The torque converter not only allows smooth gear shifting but also has other merits allowing easy and comfortable driving.
However, the main reason why automatic transmissions are inferior to manual transmissions in terms of fuel economy lies in the basic hydrokinetic operational principle of the torque converter that can never be an object of alteration or improvement.
To make the fuel economy of automobiles equipped with automatic transmissions reach or surpass that of the automobiles equipped with manual transmissions, a great improvement of the gear train is indispensable to compensate for the shortcoming of the torque converter.
When a car cruises at a constant speed, the torque converter does not cause any power loss because there is no slip, due to the operation of a torque converter clutch, between an impeller and a turbine in the torque converter.
However, when a car moves at low and/or unsteady speed as in congested cities, a torque converter clutch does not operate steadily and consequent slips between the impeller and the turbine in the torque converter make the fuel economy poorer.
Then, to attain a better fuel economy with the automatic transmission, the gear train must be improved so that operation range of the torque converter clutch may be expanded.
To realize such an improvement, step ratios between lower speed ratios have to be remarkably diminished compared to those of existing automatic transmissions.
Apart from the consideration about the torque converter, a best way of improving the fuel economy of automobiles is to increase the number of speed ratios on condition that compactness of the gear train is not seriously harmed.
Then, a desirably improved gear train will be required to provide a large number of speed ratios with considerably small step ratios between lower speed ratios.
A best way to satisfy above requirement will be to make the gear train provide a large number of speed ratios with uniform step ratios.
Such a solution well suits the knowledge that uniform step ratios are most efficient in eliminating less sufficient matches between engine speeds and desired car speeds.